pragma solidity ^0.5.16;

import "./SignedSafeMath.sol";

// TODO: Test suite

// https://docs.synthetix.io/contracts/SignedSafeDecimalMath
library SignedSafeDecimalMath {
    using SignedSafeMath for int;

    /* Number of decimal places in the representations. */
    uint8 public constant decimals = 18;
    uint8 public constant highPrecisionDecimals = 27;

    /* The number representing 1.0. */
    int public constant UNIT = int(10**uint(decimals));

    /* The number representing 1.0 for higher fidelity numbers. */
    int public constant PRECISE_UNIT = int(10**uint(highPrecisionDecimals));
    int private constant UNIT_TO_HIGH_PRECISION_CONVERSION_FACTOR = int(10**uint(highPrecisionDecimals - decimals));

    /**
     * @return Provides an interface to UNIT.
     */
    function unit() external pure returns (int) {
        return UNIT;
    }

    /**
     * @return Provides an interface to PRECISE_UNIT.
     */
    function preciseUnit() external pure returns (int) {
        return PRECISE_UNIT;
    }

    /**
     * @dev Rounds an input with an extra zero of precision, returning the result without the extra zero.
     * Half increments round away from zero; positive numbers at a half increment are rounded up,
     * while negative such numbers are rounded down. This behaviour is designed to be consistent with the
     * unsigned version of this library (SafeDecimalMath).
     */
    function _roundDividingByTen(int valueTimesTen) private pure returns (int) {
        int increment;
        if (valueTimesTen % 10 >= 5) {
            increment = 10;
        } else if (valueTimesTen % 10 <= -5) {
            increment = -10;
        }
        return (valueTimesTen + increment) / 10;
    }

    /**
     * @return The result of multiplying x and y, interpreting the operands as fixed-point
     * decimals.
     *
     * @dev A unit factor is divided out after the product of x and y is evaluated,
     * so that product must be less than 2**256. As this is an integer division,
     * the internal division always rounds down. This helps save on gas. Rounding
     * is more expensive on gas.
     */
    function multiplyDecimal(int x, int y) internal pure returns (int) {
        /* Divide by UNIT to remove the extra factor introduced by the product. */
        return x.mul(y) / UNIT;
    }

    /**
     * @return The result of safely multiplying x and y, interpreting the operands
     * as fixed-point decimals of the specified precision unit.
     *
     * @dev The operands should be in the form of a the specified unit factor which will be
     * divided out after the product of x and y is evaluated, so that product must be
     * less than 2**256.
     *
     * Unlike multiplyDecimal, this function rounds the result to the nearest increment.
     * Rounding is useful when you need to retain fidelity for small decimal numbers
     * (eg. small fractions or percentages).
     */
    function _multiplyDecimalRound(
        int x,
        int y,
        int precisionUnit
    ) private pure returns (int) {
        /* Divide by UNIT to remove the extra factor introduced by the product. */
        int quotientTimesTen = x.mul(y) / (precisionUnit / 10);
        return _roundDividingByTen(quotientTimesTen);
    }

    /**
     * @return The result of safely multiplying x and y, interpreting the operands
     * as fixed-point decimals of a precise unit.
     *
     * @dev The operands should be in the precise unit factor which will be
     * divided out after the product of x and y is evaluated, so that product must be
     * less than 2**256.
     *
     * Unlike multiplyDecimal, this function rounds the result to the nearest increment.
     * Rounding is useful when you need to retain fidelity for small decimal numbers
     * (eg. small fractions or percentages).
     */
    function multiplyDecimalRoundPrecise(int x, int y) internal pure returns (int) {
        return _multiplyDecimalRound(x, y, PRECISE_UNIT);
    }

    /**
     * @return The result of safely multiplying x and y, interpreting the operands
     * as fixed-point decimals of a standard unit.
     *
     * @dev The operands should be in the standard unit factor which will be
     * divided out after the product of x and y is evaluated, so that product must be
     * less than 2**256.
     *
     * Unlike multiplyDecimal, this function rounds the result to the nearest increment.
     * Rounding is useful when you need to retain fidelity for small decimal numbers
     * (eg. small fractions or percentages).
     */
    function multiplyDecimalRound(int x, int y) internal pure returns (int) {
        return _multiplyDecimalRound(x, y, UNIT);
    }

    /**
     * @return The result of safely dividing x and y. The return value is a high
     * precision decimal.
     *
     * @dev y is divided after the product of x and the standard precision unit
     * is evaluated, so the product of x and UNIT must be less than 2**256. As
     * this is an integer division, the result is always rounded down.
     * This helps save on gas. Rounding is more expensive on gas.
     */
    function divideDecimal(int x, int y) internal pure returns (int) {
        /* Reintroduce the UNIT factor that will be divided out by y. */
        return x.mul(UNIT).div(y);
    }

    /**
     * @return The result of safely dividing x and y. The return value is as a rounded
     * decimal in the precision unit specified in the parameter.
     *
     * @dev y is divided after the product of x and the specified precision unit
     * is evaluated, so the product of x and the specified precision unit must
     * be less than 2**256. The result is rounded to the nearest increment.
     */
    function _divideDecimalRound(
        int x,
        int y,
        int precisionUnit
    ) private pure returns (int) {
        int resultTimesTen = x.mul(precisionUnit * 10).div(y);
        return _roundDividingByTen(resultTimesTen);
    }

    /**
     * @return The result of safely dividing x and y. The return value is as a rounded
     * standard precision decimal.
     *
     * @dev y is divided after the product of x and the standard precision unit
     * is evaluated, so the product of x and the standard precision unit must
     * be less than 2**256. The result is rounded to the nearest increment.
     */
    function divideDecimalRound(int x, int y) internal pure returns (int) {
        return _divideDecimalRound(x, y, UNIT);
    }

    /**
     * @return The result of safely dividing x and y. The return value is as a rounded
     * high precision decimal.
     *
     * @dev y is divided after the product of x and the high precision unit
     * is evaluated, so the product of x and the high precision unit must
     * be less than 2**256. The result is rounded to the nearest increment.
     */
    function divideDecimalRoundPrecise(int x, int y) internal pure returns (int) {
        return _divideDecimalRound(x, y, PRECISE_UNIT);
    }

    /**
     * @dev Convert a standard decimal representation to a high precision one.
     */
    function decimalToPreciseDecimal(int i) internal pure returns (int) {
        return i.mul(UNIT_TO_HIGH_PRECISION_CONVERSION_FACTOR);
    }

    /**
     * @dev Convert a high precision decimal to a standard decimal representation.
     */
    function preciseDecimalToDecimal(int i) internal pure returns (int) {
        int quotientTimesTen = i / (UNIT_TO_HIGH_PRECISION_CONVERSION_FACTOR / 10);
        return _roundDividingByTen(quotientTimesTen);
    }
}